Somewhat hard to say. What do you want to do afterwards?
I gather Essex has a very strong politics (presumably including IR) department with links to HKS and such I think even? If you are looking at working internationally though SOAS might have a stronger "brand name" - within the UK that doesn't really matter so much outside of investment banking/management consulting (in which case neither is a target uni for undergrad, although I think SOAS is a target for masters?).
SOAS of course has a lot of rare language options that may be very interesting or useful to take advantage of while you are there as optional modules. It also has a lot of specialism on non-Western politics and IR topics, so if that's your main interest you might have more options in that field at SOAS.